Resistor



Oct. 4, 1932. G.' B. cRousE 1,880,239

'RESISTOR Filed July 24, 1930 2 Sheets-sheet 1 BYW Patented Oct. 4,1932` UNITED STATES ,Pari-:Nrl orf-'ICE GEORGE B. CROUSE, F NEWARK, NEWJERSEY, ASSIGNOR T0 HARDWIOK, HINDLE, INC.,

OF NEWARK, NEW JERSEY, A CORPORATION OF NEW JERSEY RESISTOR Applicationled July 24, 1930. Serial No. 470,380.

This invention relates to resistors and more particularly to those ofthe tubular, vitreous enamel coatedl type.

' In general a vitreous enamel resistor comprises a tube of porcelain orother refractory material, around the ends of which are clamped suitablecontact bands of copper or other vconducting material. The resistancewire, which is usually an alloy having high electrical resistance, isthen wound on the tube between the contact bands and riveted orotherwise attached' thereto at its ends.

The wound tube is then dipped or sprayed with the vitreous enamel slipand fired at such a temperature that the enamel becomes molten and flowsover the entire surface of wire and tube. One or more coats of enamel,as required, are applied and fired over the original coat.

It is obvious that in a resistor of this type, since the entire wire iscoated with enamel, it is impossible to make any ad'ustments of theresistance value or to take o a contact at any point after the resistoris completed. This feature is disadvantageous from several angles. Inthe first place, even through the original resistor is wound accuratelyto a specified resistance value, oxidation of the wire will take placein the ring process, as a result of which the wire is reduced incrosssection and its resistance increased by an amount which is notaccurately predictable; secondly, in many applications of a resistor itis desirable to be able to change the resistance in service or to usethe device as an adjustable potentiometer.

l Many attempts have been made in the past to produce a resistor of thet pe to which this invention is directed, none o which have beenconspicuously successful. f Since the finished enamel coating is veryhard and brittle, it is impossible to satisfactorily grind it off alongthe section in which it is desired to expose the wire, and, therefore,most of the previous attacks on the problem were directed to exposing atrack before firing. The common method was as follows: Before the woundresistor was dipped or sprayed in theenamel slip, a shield, generally ofadhesive tape, was laid l along the resistor, the enamel thusbeing'prevented from reaching the area which it was proposed to expose.The disadvantages of this method are twofold In the first place, theiiuid enamel in the furnace, during the ring process, has a very highsurface tension and it therefore pulls back away from that area whichwas shielded and leaves a` larger and irregularly edged area exposed.-This produces a unit of very poor appearance and has otherdisadvantages, as will be seen immediately. Secondly, the wires areentirely exposed and unsupported in the slot. As a result of this, anyform of sliding contact that may be used with the wires tends todisplace them, bend them, and ultimately to break them.

One object of the invention is a method of producing a vitreous enamelresistor, as well as the resistor itself as an article of manufa cture,having an area in which the wires are exposed, so that contact may bemade with them at any desired point in this area. Except where the wiresare exposed, they are completely embedded in enamel.

Another object of the invention is the provision with a resistor of thetype described above of a slidable contact device whereby the resistormay be used as a rheostat or potentiometer.

Briefly described, the method consists in completely coating with theenamel slip a form on which the resistance wire has previously beenwound. For most convenient practice of the method the enamel is allowedto partially dry and the unit is then placed in a 35 device which has ashield which covers all portions of the resistor except the area whichit is desired to expose. The slip is then brushed oii of that area, theunit is removed from the device and fired in the usual way. Some pullingof the enamel may occur on the first coat, but on the second coat aclean edged exposed area results, in which only the upper surface of thewire projects from the enamel, the brushing having allowed a portion ofthe slip to remainbetween the turns.

The particular nature of the invention, as well as other objects andadvantages thereof, will appear most clearly from a descriptionofcertain preferred embodiments there- 1M device,

Fig. 3 is a transverse sectional view on an enlarged scale showing theexposed portion of the resistor, this view being taken on the line 3 3of Fig. 4,

Fig. 4 is a corresponding view, but in longitudinal section along theline 4 4 of Fig. 3,

Fig. 5 shows a modified form of tube for the resistor whereby sharperedges of the enamel along the sides of the exposed track may be secured,

Fig. 6 is a transverse view of Fig. 5,

Fig. 7 is an enlar ed transverse sectional view of the expose part of aresistor in which the tube of Figs. 6 and 7 is used,

Fig. 8 is an elevation view of the complete the tube of resistorarranged 'as a rheostat,

Fig. 9 is a plan view of the same, and F-ig.o 10 is a transversesectional view taken along the line 10-10 of Fig. 8.

`In practicing the method of the invention a form l, such as a tube ofrefractory 'material, is wound with suitable wire 2, the

ends of which are attached to contact bandsl 3, the ends of which extendoutwardly to serve as terminals 4 and are suitably fastened together, asshown in Fig. 2. The

resistance unit thus produced is completely coated with suitable enamelslip 5 in the usual way and then allowed to partially dry in the air.

The unit is then placed in the device shown in Figs. 1 and 2. The deviceconsists of a base 6 provided with standards 7 and 8 at its ends.Standard 7 has an inwardly extending conical centering member 9 adaptedto enter one end of the tube 41. A rod 10, provided with a knob 11 isslidably mounted in the other standard 8. To its inner end is attached asimilar centering member 12 ,adapted to enter the other end of the tube1 and be held therein by a spring 13 surrounding the rod between thestandard 8 and the centerin member 12. By means of knob 11 the mem ermay be retracted to permit the unit to be placed in or removed from thedevice. To aid in properly positioning the unit in the device, the baseis provided witha projection 14 against which one of the terminals 4 maybe placed.

A shield 15 is hinged upon a rod 16 extending between the standards 7and 8 and is provided with a slot 17 of a width correspondingsubstantially to that of the desired width of the exposed area on theresistor, the sides'of the slot being preferably' bevelled as shown mostclearly in 2. In order to revent the slide from coming in contact withthe damp enamel slip and sticking to it, adjustable set screws 18 areplaced at the outer corners of the shield 15 and rest upon lugs 19projecting inwardly from the standards 7 and 8.

After the partially dried unit has been placed in the device with theshield lifted, the latter is lowered until the set screws 18 engage thelugs 19, the screws being properly adjusted to hold the shield close tobut out of contact with the enamel on the unit.

The unit being in place between the centering members and the shieldnlowered over it, the enamel slip is brushed off by passing a bristlebrush along the`slot in the shield. Thel unit is then removed and isready to be fired. The required number of coats having been applied inaccordance with the procedure described above, it will be found thatthere is a very thin skin of oxide and enamel on the exposed portions ofthe wire. This may be removed by placing the unit in the same, or in asimilar, shield device and rubbing the exposed portion with emery clothorother suitable abrasive. Very 'little effort is re uired to give abright surface to the expose wire in the track 20.

The appearance of the resulting resistor is best illustrated in Figs. 3and 4 wherein the exposed outer surfaces of the wire 2 between the edgesof the enamel 5 at the sides of the track 20 are shown as projectingslightly above the enamel which during the brushing operation hasremained between the turns of the wire. This enamel after the iiringoperation keeps the turns properly spaced and holds them firmly againstdisplacement when a slidable Contact device is used with the wire asWill presently be described.

A further sharpening of the edge of the enamel along the sides of theexposed track may be secured by the use of longitudinal slots in therefractor tube as shown in Figs. 5 and 6. In these gures the tube isindicated at 1 with the two slots 21. The particular shape of theseslots is not important, but they are so located that when the wound unitis placed in the device, these slots come ateither edge of the areaswhich it is proposed to expose by brushing. In other respects theprocess remains exactly the same. While the use of these slots isbeneficial, they are not essential, and commercially satisfactory unitscan be produced without them. Their action, apparently, is to serve as asmall reservoir of fiuid enamel in the furnace and thus prevent thesurface tension from pulling the enamel away from the exposed track. Theresulting resistor is shown in Fig. 7 which shows how the enamel hasflowed into the slots 21.

In the manufacture of resistors of the type described in thisspecification, it is found wire will expand away from the tube in thering process and the surface tension of the enamel may pull the turnsYapart in some -places and bunch them together in others,

making an uneven spacing of turns in the finished unit.

The resistor produced as described above is particularly adapted for useas the resistance unit of a rheostat or potentiometer. Such anapplication of the invention is shown in Figs. 8, 9, and 10. ,Theresistor indicated generally by 22 is clamped between two end members 23by a rod 24 with washers 25 interposed between the resistor and themembers to take care of relative expansion between the metal of the rodand the material of the tube. The end members serve as supports for thedevice andalso carry between them aI rod 26, preferably of rectangularcross section and insulated from the members by bushings 27. The rod is-held in place by nuts 28 on its threaded ends, one of the ends beingprovided with binding post elements 29. The terminals 4 of the resistorare also provided with -binding posts 30 for connections to an externalcircuit.

Slidably mounted on the rod 26 is a contact device 31 of the followingconstruction:

A tubular member 32, preferably of rectangular shape, is vplaced overthe rod 26, and a spring 33 is placed between the bottom of the tubularmember and the lower side of the rod, the free end of the spring bearingl a upon the exposed track on the resistor. A

metallic block 34 is placed between the top of the tube and the upperside of the rod. F or the purpose of adjusting the Contact device amember 34 of insulating material is placed over the tube and the blockand is connected to these elements by a screw 35 having its head set ina depression in the member to avoid danger of contact with the fingersof the operator when the device 1s moved along the rod to adjust itsposition relatively to the exposed track.

While certain preferred embodiments of lthe invention have beendisclosed, it will be understood that it `may be emplo ed in theproduction of resistors of various orms having exposed areas of variousshapes and that various changes in the details of construction of thedevices shown herein as well as modifi-4 cations of the method describedherein may be made without departing from the principles of invention asdefined in the appended claims.

I claim:

. l. The method of producing a resistor which consists in winding a wireupon a V refractory form, completely coating the wire and form withvitreous enamel slip, brushing olf the slip over an area to expose thewlre and subjecting the slip to high temperature to lire the same.v

2. The method of vproducing a, resistor which consists in winding a wireupon a refractory form, completely coating the wire andform withvitreous enamel'slip, allowing the slip to partially dry, brushing oil'the partially dried slip over an area to expose the wire and subjectingthe slip to high temperature to fire the same.

3. The method of producing a resistor which consists in windinga wireupon a refractory form, under high tension, completely coating the wireand form with vitreous enamel slip, brushing oiii' the slip over an areato expose the wire and sub]ecting the slip to high temperature to lirethesame.

4. The method of producing a resistor which consists in winding a wireupon a refractory form, completely coating the wire and form withvitreous enamel slip, brushing off the slip from the outer portions ofthe turns ofwire over an area to expose the wire while leaving some ofthe slip between the lturns and subjecting the slip to high temperatureto fire the same.

5. The method of reducing a resistor which consists in lwinding a wireupon a refractory form, completely coating the wire and form withvitreous enamel slip, brushing oft' the slip from a longitudinal trackto expose the wire and subjecting the slip to high temperature to firethe same.

In testimony whereof I aliix m signature.

' GEORGE B.- ROUSE.

